1. Field of the Invention
The invention relates to a spark plug improved so as to effectively prevent flashover from occurring behind a front end surface of an insulator when applying a high voltage across electrodes at the time of ignition.
2. Description of Prior Art
In a spark plug in which spark discharges are induced within an air-fuel mixture gas, an elongated center electrode is placed in a tubular insulator, and a metal shell which supports the insulator, and a front end portion of the center electrode faces a front end of an outer electrode to induce spark discharges across a spark gap provided therebetween.
In this type of the spark plug as represented by FIG. 7, the center electrode has a columnar section 100 on which a semi-frusto-cone shaped step section 101 is provided which is tapered off toward a front end of the center electrode in order to improve an ignitability. On a front end surface of the step section 101, a small columnar portion 102 is provided to reach a front end of the center electrode. It is well-known that the small columnar portion 102 is made of a noble metal material such as a Pt--Ir alloy or the like.
As foreign prior art references relevant to the present invention, U.S. Pat. Nos. 4,914,344 and 4,845,400 are thus far introduced.
However, in the case in which a boundary 103 between the columnar section 100 and the the step section 101 is protracted outside of a front end surface 104 of the insulator as shown in FIG. 7, equi-potential curves concentrate on the front end surface of the insulator so as to likely invite the flashover from the boundary 103 toward a shoulder portion 84 of the insulator as shown at (Fo) in FIG. 7, instead of normally inducing spark discharges across a spark gap between the center electrode and the ground electrode.
This is all the more true for a spark plug which is incorporated into a gas engine which uses gaseous fuel such as natural gas, synthetic gas, LPG or the like. Because this type of the spark plug employs a shorter leg portion of the insulator with a high compression ratio so that a high spark voltage is required to likely invite the flashover.
In the prior art reference of U.S. Pat. No. 4,914,344, a boundary positions inside a front end surface of an insulator 3. However, the structure is such as to induce capacitor discharges between a forward end 41 of third electrode 4 and a front end of the center electrode 1 by way of an auxiliary gap S2, thus likely to invite the flashover behind a front end surface of the insulator.
In the prior art reference of U.S. Pat. No. 4,845,400, a boundary and a taper portion 31 itself are located inside a front end surface of the insulator 2. This provides a wide space between the taper portion 31 and a front inner wall of the insulator 2. The structure is such that a greater amount of heat is retain within the wide space, thus quickly deteriorating a front end of a center electrode.
Therefore, it is a main object of the invention to provide a spark plug which is capable of positively preventing the flashover from occurring behind the front end surface of the insulator, thereby normally inducing spark discharges in the air-fuel mixture gas within a wide range of operating conditions.